The present invention relates to a wafer heating apparatus mainly used for heating a wafer.
Wafer heating apparatus are used for supporting wafers on their mounting plates, such as semiconductor wafers, liquid crystal substrates or other circuit substrates (hereinafter called simply “wafer”), and heating conductive and insulating films applied onto the wafer, forming circuit elements thereon, or for forming light sensitive resist films by drying and heating liquid films applied onto the wafers.
For example, in the semiconductor device fabrication industry, wafer heating apparatuses have been used to heat semiconductor wafers in the processes of formation of conductive or insulating films, photoresist films, or of etching treatments of such films.
In conventional apparatuses, a batch type of apparatus has been used which has performed film treatment of a plurality of wafers collectively at one time. However, as wafers have, recently, increased from 8 to 12 inches in diameter, there has been used an apparatus which heat treats a single wafer at one time to improve its processing precision. However, this single-wafer treatment type of apparatus limits the number of wafers to be processed at one time to only one, and thus, requires shortened time taken for treating the single wafer. For that reason, the wafer heating apparatus is required to achieve an improvement in the precision of the heating temperature as well as reduction of heating time and prompt placing and displacing of the wafer on the apparatus.
As an example of the wafer heating apparatus as mentioned above, a wafer heating apparatus 21 shown in JP-A-11-283729, for example, is exemplified. As shown in FIG. 8, the wafer heating apparatus 21 is mainly composed of a casing 31, a ceramic plate 22 and a stainless plate 33 as a reflection plate. The casing 31 is a bottomed metallic member (which is herein an aluminum member) and its upper portion is provided with an opening 34 that is circular in cross section. A central portion of this casing 31 is formed with three holes 35 for passing through wafer-supporting pins (not shown). If the wafer-supporting pins passed through the holes 35 are moved up and down, it is possible to pass a wafer W to a carrier machine or possible to receive a wafer W from the carrier machine. In a conductive terminal portion for a resistance heating element 25 shown in FIG. 9, conductive terminals 27 are brazed. The conductive terminal portion is so constructed that the conductive terminals 27 are passed through holes 57 formed in the stainless plate 33. Several lead extraction holes 36 are formed in an outer circumferential portion of the bottom 31a. Leads not shown for supplying a resistance heating element with electric current are passed through the holes 36, and said leads are connected to the conductive terminals 27.
As ceramic materials constituting a ceramic plate 22, nitride ceramics or carbide ceramics are used, and, as shown in FIG. 9, there is proposed the wafer heating apparatus 21 which energizes a plurality of concentrically formed patterns to heat the ceramic plate 22.
In such a wafer heating apparatus 21, it is important to control the wafer W at a uniform temperature as well as to accurately measure the temperature of the wafer W in order to heat a resist film uniformly. Therefore, temperature sensors for measuring temperatures of the wafer W are used, and the temperature sensors are fitted to recesses 23 of the wafer heating apparatus.
JP-A-9-45652 discloses a method of disposing a temperature sensor 150 which measures a temperature of a wafer W placed on a wafer heating apparatus and controlling the temperature of a front surface 40a of a metal plate 40 is controlled as shown in FIG. 10. As the method of enhancing precision in temperature control attributable to precision in the temperature of the plate 40 and the response thereof, there is disclosed a method in which a temperature difference in the longitudinal direction of the temperature sensor 150 is made smaller and the temperature sensor 150 is disposed in a manner so as to be parallel to the upper surface of the plate 40. In the temperature sensor, the temperature sensor 150 made of Pt is inserted into a protective tube 151 and disposed to be parallel to the front surface 40a of the plate 40.
Further, a clearance in the protective tube 151 is filled with thermal conductive cement 52. In particular, in the case where a resistance heating element is divisionally controlled, precise temperature control of the plate 40 cannot be achieved unless measurement variations as well as precise measurement are controlled. Therefore, applying such a fitting structure was considered to be preferred.
JP-A-4-98784 discloses that, in a wafer heating apparatus wherein a single resistance heating element is buried in a ceramic plate, temperature-measuring points are positioned on the wafer heating surface of the ceramic plate at a distance of about 1/√2 times a radius of a wafer heating region on the ceramic plate from the center of the wafer heating region in order to prevent the temperatures on a heating region for heating the wafer from deviating from an optimal value.
JP-A-2001-85144 discloses a wafer heating apparatus 21 as shown in FIG. 9 wherein, as a temperature sensor, a thermocouple with a wire size of less than 0.5 mm is inserted in a recess 23, having a depth of 2 mm and a diameter of 1.2 mm, formed in the ceramic plate 22 of a thickness of 3 mm, which recess is then sealed with a heat-resistant resin.
In JP-A-2002-100559 or JP-A-2001-338862, a wafer heating apparatus is described in which a recess is formed in the ceramic plate and then a temperature sensor is press-fixed with a fixing member.
However, it is required that, in a ceramic plate used for a wafer heating apparatus that processes wafers one by one, which has recently attracted public attention, its thickness is required to be reduced to 2-5 mm in order to shorten the processing time for one wafer. Thus, the processing time must be adjusted so that a cycle time for heating and cooling is shortened. However, in order to uniformly heating the whole surface of a wafer in the range of plus or minus 0.5° C., there was a problem that the object of uniformly heating the wafer cannot be achieved only by providing a temperature sensor in a ceramic plate in a conventional manner.
In the wafer heating apparatus, even if the temperature sensor 150 is disposed in a manner so as to be parallel to the mounting surface 40a of the plate 40, on which a wafer W is mounted, as disclosed in JP-A-9-45652, the plate 40 made of metal has a large thickness of 30 mm or more and thus it was not possible to raise or reduce the temperature of the plate 40 rapidly. Furthermore, heat is dissipated to the outside of the plate 40 from the body of the temperature sensor 150 or the connecting member to the temperature sensor 150, so that the temperatures of the temperature measuring portions are lowered, or the temperature sensor 150 cannot surely be thermally connected to the upper surface of the recess 41 of the plate 40. Therefore, there was a problem that the temperatures of the plate 40 and the wafer W cannot accurately be measured.
The temperature of the wafer W has poor responsiveness to the temperature set depending on the distance from the resistance heating element fitted to the plate 40 or the distance from the mounting surface 40a to the temperature sensor 150 and thus the temperature varies, and it takes a long time to control the Wafer W at a uniform temperature. Thus, there was a problem that the processing time for the wafer increased.
Furthermore, a wafer heating apparatus wherein a thermocouple is connected to the ceramic plate described in the above JP-A-2002-100559 and JP-A-2001-338862 is easily affected by noise due to a change in the outside temperature and so on and thus there was difficulty in controlling the wafer W into an extremely uniform temperature for example, within a temperature difference of less than 0.1° C.